On the Happijacks, if you look at the front turnbuckles, you'll notice a line around the hooked end that attaches to the truck (actually, you won't see it until you start tightening the turnbuckle). I was advised to tighten until the line is about a 1/4" from the cylinder.
For the rears, I was taught to tighten just until you feel the resistance from contact being made with the rubber component inside the cylinder (forgive my highly un-technical description).
I can't say for sure if the cabover struts would work for you, but we have been using them for approaching 14-years, on two different rigs, and love them. Porpoising is greatly reduced and as our dealer told us from the beginning - with struts you will still get the first bounce, but not the second or third.